Mechanochemical process to produce exfoliated nanoparticles

ABSTRACT

The invention relates to a mechanochemical process to produce exfoliated nanoparticles comprising the steps of
         providing a solid feedstock comprising a carbonaceous and/or mineral-based material;   providing a flow of an oxidizing gas;   introducing the solid feedstock and the flow of an oxidizing gas into a mechanical agitation unit,   subjecting the material of the solid feedstock in the presence of the oxidizing gas to a mechanical agitation operation in the mechanical agitation unit at a pressure of at least 1 atm (15 psi).
 
The invention further relates to nanoparticles obtainable by the mechanochemical process and to the use of such nanoparticles.

FIELD OF THE INVENTION

The present invention relates to a mechanochemical process to produceexfoliated nanoparticles. The invention further relates to thenanoparticles obtainable by this mechanochemical process and to the useof the nanoparticles.

BACKGROUND ART

During recent years graphene and graphitic nanoplatelets emerged as avery promising material because of its unique combination of properties,opening a way for their exploration in a wide spectrum of applications.

Presently known methods to produce graphene comprise techniques based onchemical cleavage or mechanical cleavage, chemical vapour depositiontechniques, epitaxial growth methods, liquid phase exfoliationtechniques.

Some of these techniques use toxic chemicals or create hazardous wasteor poisonous gases and can therefore not be considered as valuabletechniques to produce graphene.

Furthermore it remains challenging to find a method that allows theupscaling of graphene production resulting in high quality graphene,produced at reasonable costs and in a reproducible manner.

Therefore, there is a need to provide an improved method to providegraphene, nanoplatelets and graphitic nanoplatelets.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method to produceexfoliated nanoparticles, as for example exfoliated graphenenanoparticles.

It is another object to provide a method to produce exfoliatednanoparticles that can be upscaled.

It is a further object of the invention to provide a method that doesnot require toxic chemicals.

Furthermore it is an object of the invention to provide a method thatallows to sequester carbon dioxide (CO₂), for example from CO₂emissions, in a stable solid form that is suitable in a number ofdifferent applications.

It is still a further object of the invention to create nanoparticlesusing waste CO₂ and cheap solid feedstock.

According to a first aspect of the present invention, a mechanochemicalprocess to produce exfoliated nanoparticles is provided. The processcomprises the steps of

-   -   providing a solid feedstock comprising a carbonaceous and/or        mineral based material,    -   providing a flow of an oxidizing gas, as for example a flow of        carbon dioxide,    -   introducing the solid feedstock and the flow of an oxidizing gas        into a mechanical agitation unit,    -   subjecting the solid feedstock in the presence of the oxidizing        gas to a mechanical agitation operation in the mechanical        agitation unit at a pressure higher than 1 atm (15 psi).

In particular, the process according to the present invention allows toupcycle emissions, such as CO₂ emission and solid feedstock, such ascarbonaceous feedstock to produce nanoparticles as for example graphenenanoparticles.

According to a second aspect of the present invention, nanoparticlesobtainable by the above described mechanochemical process are provided.It has been established by the present inventors have, in variousrespects, superior properties over existing materials. Accordingly thenanoparticles of the present invention confer significant benefits in alarge variety of applications.

According to a third aspect of the present invention, the use ofnanoparticles obtainable by the above described mechanochemical processis provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be discussed in more detail below, withreference to the attached drawings in which

FIG. 1 depicts a flowchart of a method according to the presentinvention;

FIG. 2 shows the Young's modulus (A and B), peak stress (C and D),resilience (E and F) and maximum elongation (G and F) for HDPE-GNP andHDPE-GO composites;

FIG. 3 shows the storage modulus (A and B), loss modulus (C and D), losstangent (E and F) for HDPE-GNP and HDPE-GO composites;

FIG. 4 shows the Differential Scanning calorimetry thermograms forHDPE-GNP and HDPE-GO composites.

FIG. 5 shows the cell viability results of different graphenederivatives at different concentrations (μg/mL).

DESCRIPTION OF EMBODIMENTS

The process according to the present invention to produce exfoliatednanoparticles is illustrated in FIG. 1 .

A solid feedstock 11 and a flow of oxidizing gas 12 are introduced in amechanical agitation unit 13. The material of the solid feedstock 11 isin the presence of the oxidizing gas subjected to a mechanical agitationoperation in the mechanical agitation unit 13 at a pressure of at least1 atm (15 psi). More preferably, the mechanical agitation unit 13 ispressurized at a pressure of at least 2 atm (29.4 psi).

For the purpose of the present invention a carbonaceous material isdefined as a material comprising carbon or carbon compounds. Examples ofa feedstock comprising a carbonaceous material comprise fly ash, bottomash (incinerator ash), graphite, petroleum coke, anthracite coal,bituminous coal, activated carbon, charcoal, or combinations thereof.

For the purpose of the present invention a mineral-based materialcomprises inorganic materials as for examples oxides and silicates.Examples of a feedstock comprising an inorganic material compriseolivine, talc, yellowstone, serpentine, saw dust or amorphous powder orcombinations thereof.

In accordance with the invention, the process can be performed simply bytreating the feedstock in a solid, dry state. Hence in accordance withembodiments of the invention, a process as defined herein is provided,wherein the solid feedstock comprises less than 10% of water, based onthe total weight of the feedstock, e.g. less than 8%, less than 6%, lessthan 5%, less than 4%, less than 3%, less than 2%, less than 1%, lessthan 0.5% or less than 0.1%. In accordance with embodiments of theinvention, a process as defined herein is provided, wherein the processdoes not comprise any step wherein substantial amounts of water orsolvent are added to the feedstock. In preferred embodiments of theinvention, no water or solvent is added to the feedstock at all. Inother embodiments of the invention, less than 10% of water and/orsolvent are added, based on the total weight of the feedstock, e.g. lessthan 8%, less than 6%, less than 5%, less than 4%, less than 3%, lessthan 2%, less than 1%, less than 0.5% or less than 0.1%.

The flow of oxidizing gas may comprise any type of oxidizing gas as forexample oxygen, sulfur dioxide, nitrogen dioxide and carbon dioxide. Apreferred oxidizing gas comprises carbon dioxide (CO₂). The carbondioxide may for example comprise carbon dioxide (CO₂) emissions fromburning of fossil fuels like coal, gas or oil, or carbon dioxideemissions from industrial processes such as cement manufacturing.

One of the advantages of the present invention, compared to processesknown in the art, resides in the fact that the process can be performedwith oxidizing gas, such as carbon dioxide gas, of relatively lowpurity. It is feasible, example, to use carbon dioxide gas emissionsderived from industrial processes, i.e. as a waste or side stream. Hencein preferred embodiments of the invention, processes as defined hereinare provided wherein a gas is used that is relatively impure, e.g. a gascomprising carbon dioxide at a level within the range of 70-95%, e.g.70-90%, 70-85%, 75-95%, 75-90%, 75-85%, 80-95% or 80-90%.

A further advantage of the present invention, compared to processesknown in the art, resides in the fact that the process does not requirethe oxidizing gas to be in a supercritical state. Hence, the solidfeedstock can be treated with the oxidizing gas without having to applypressures and temperatures high enough to bring and/or keep theoxidizing gas in a supercritical state. Hence, in accordance with theinvention, a process is provided as defined herein, wherein theoxidizing gas is in a gaseous state, as will be evident from the generaldescription of the process and the specific process conditions as wellas from the appending examples. In accordance with the invention, theoxidizing gas is typically not in the supercritical state.

For the purpose of this invention the mechanical agitation operation maycomprise any method to apply kinetic energy to the solid feedstock tofacilitate the interaction with the flow of the oxidizing gas. Examplesof mechanical agitation include mixing, stirring, shearing, shaking,blending, ultrasonication and combinations thereof. Examples of shearingcomprise low-torque or high torque shearing. Examples of stirringcomprise low speed or high speed stirring, centrifuging or sonication.

A particular advantage of the present invention, compared to processesknown in the art, resides in the fact that the kinetic energy requiredto successfully carry out the process is relatively low. Hence inpreferred embodiments of the invention, processes as defined herein areprovided wherein the kinetic energy applied is less than 5 MW per ton ofend product produced, preferably less than 4 MW per ton, more preferably3 MW per ton.

It can be preferred to add a catalyst to the solid feedstock. Preferredcatalysts comprise metal oxides, as for example iron oxides, cobaltoxides, rhenium oxides, titanium oxides and combinations thereof. Thecatalyst can be added through a lining for example a lining on theinside wall of the mechanical agitation unit or on a component of themechanical agitation unit. The lining comprises for example a sputteredlining. Alternatively, the catalyst can be added through solution mixingwith the material of the feedstock.

In preferred mechanochemical processes according to the presentinvention an intercalant agent is added during the process. Preferredintercalant agents comprises acids such as hydrochloric acid, sulfuricacid or nitric acid.

The intercalant agent can be added before, during or after themechanical agitation operation of the material of the solid feedstock.

Preferably, the mechanical agitation unit is pressurized at a pressureof at least 1 atm (15 psi). More preferably, the mechanical agitationunit is pressurized at a pressure of at least 2 atm (29.4 psi).

The nanoparticles obtainable by the mechanochemical process according tothe present invention may comprise nanosheets, nanoparticles ornanoplatelets.

The nanoparticles preferably have a BET surface ranging between 10 m²per gram and 1000 m² per gram, more preferably between 50 m² per gramand 1000 m² per gram as for example 100 m² per gram, 200 m² per gram,300 m² per gram, 500 m² per gram, 600 m² per gram, 700 m² per gram, 800m² per gram, 900 m² per gram or 950 m² per gram.

The nanoparticles have preferably a D50 particle size distributionranging between 20 nm and 10 μm and more preferably between 50 μm and 5μm as for example 100 nm, 200 nm, 300 nm, 500 nm, 1 μm, 2 μm or 3 μm.The D50 particle size distribution is defined as the median diameter orthe medium value of the particle size distribution, it is the value ofthe particle diameter (or the particle equivalent diameter) at 50% inthe cumulative distribution.

The nanoparticles obtainable by the mechanochemical process according tothe present invention may have a bimodal distribution with a first setof nanoparticles and a second set of nanoparticles. The first set ofnanoparticles has preferably a particle size ranging between 50 nm and300 nm and more preferably between 100 nm and 300 nm, whereas the secondset of nanoparticles preferably has a particle size ranging between nm 1μm and 10 μm and more preferably between 1 μm and 5 μm. The bimodaldistribution is achieved through the optimal use of the catalystsutilized in the exfoliation process.

In embodiments of the invention the nanoparticles obtainable by themechanochemical process have a bimodal distribution with a first set ofnanoparticles having a D50 within the range between 50 nm and 300 nm andmore preferably within the range of 100 nm and 300 nm, and a second setof nanoparticles having a D50 within the range of 1 μm and 10 μm andmore preferably within the range of 1 μm and 5 μm.

In preferred embodiments of the invention, particle sizecharacteristics, such as D50 values, are determined using a dynamiclight scattering method with an ethanol dispersion of between 0.1 and 5mg/mL concentration.

In case the solid feedstock comprises a carbonaceous feedstock, thenanoparticles obtainable by the above described mechanochemical processhave preferably a C/O ratio ranging between 1 and 40, for example a C/Oratio of 5, 10, 20, 25, 30 or 35.

The nanoparticles obtainable by the above described mechanochemicalprocess preferably show a release of CO₂ ranging between 5 and 35 wt %of the mass of the nanoparticles. The CO₂ is released by exposing thenanoparticles to elevated temperatures. CO₂ release is initiated at atemperature between 180 and 200° C. for carbonaceous feedstock and at atemperature of 600° C. for mineral-based feedstock and continues toincrease as the ambient temperature is raised. The CO₂ release can betuned based on the temperature. For carbonaceous feedstock, only 20% ofthe entrained CO₂ is released before 300° C. in optimized production,and 100% of the CO₂ is released when the temperature is raised to 600°C. For mineral-based feedstock, the product releases 100% of theentrained CO₂ by 800° C.

The nanoparticles obtainable by the above described mechanochemicalprocess are suitable to be used as additive, for example as additive topolymer materials. The nanoparticles can be used as an additive forthermosetting materials, for example epoxies or as an additive forthermoplastic materials, for example polyethylene, in particular highdensity polyethylene (HDPE), low density polyethylene (LDPE), linear lowdensity polyethylene (LLDPE), medium density polyethylene (MDPE),polypropylene, thermoplastic polyurethane, polyamides such as nylon 6 ornylon 6 6. The nanoparticles are also suitable to be added tobiodegradable polymer materials such as polyhydroxyalkanoate (PHA) orpolylactic acid (PLA). By adding nanoparticles to the polymer material,the properties of the polymer material may be influenced. Thenanoparticles can for example be added to amend one or more of thefollowing properties:

-   -   to increase the tensile strength and/or the tensile modulus of        the base resin of the polymer material,    -   to increase the UV-resistance of the base resin of the polymer        material,    -   to increase the crystallization temperature of the base resin of        the polymer material,    -   to increase the coefficient of friction of the base resin of the        polymer material,    -   to influence the surface properties of the base resin of the        polymer material and thus the behaviour of the polymer material        in wet service in water or other fluids, for example by        influencing the hydrophilicity of the surface of the base resin        of the polymer material,    -   to influence the thermal conductivity of the base resin of the        polymer material,    -   to produce a nanocomposite with potential for dew nucleation,        promotion of vapour condensation, and air filtration of        moisture,    -   to decrease the gas permeability of a gas, for example to        decrease the gas permeability of methane, ethane, hydrogen or        oxygen.

The nanoparticles can for example be added in a concentration rangingbetween 0.05 wt % and 10 wt %.

The nanoparticles obtainable by the above described mechanochemicalprocess are also suitable as additive to a coating, such as polymercoatings, metal coatings, epoxy coatings or inorganic coatings. Thenanoparticles can be added to influence properties of the coating suchas the corrosion resistance or lubricity of the coating. Thenanoparticles can for example be dispersed effectively in ceramicchemistries, such as magnesium oxide coatings for nano-levelcrosslinking, better lubricity, and higher corrosion resistanceparticularly at elevated temperatures.

The nanoparticles obtainable by the present invention can be added toadhesives, for example polyurethane-based adhesives. By addingnanoparticles according to the present invention to adhesives as forexample polyurethane-based coating the lap shear strength and thepull-off strength may be increased. The lap shear strength may bedoubled and the pull-off strength may be increased with for example by30%.

Nanoparticles can be used as additive in inks, as for example conductiveinks such as conductive inks for printed electronics.

The nanoparticles are furthermore suitable as additive in cementitiousmaterials. By adding the nanoparticles to cementitious materials themicro- and macro-properties of cementitious material can be influenced.Addition of nanoparticles to cementitious materials as for examplemortar may enhance the compressive strength. Furthermore addition ofnanoparticles according to the present invention may decelerate thechemical attack for example induced by an acidic solution and mayimprove the corrosion resistance of cementitious material. Furthermorethe addition of nanoparticles such as graphene nanoparticles or grapheneoxide nanoparticles may promote the interfacial bond between thenanoparticles and the carbon silicate hydrate gels (C-S-H gels) aroundthem. Furthermore by adding nanoparticles to asphalt cement, theproperties such as the freeze thaw resistance of the asphalt mix can beimproved.

Nanoparticles according to the present invention can be added tomembranes, for example polymer membranes or can be added to a coatingapplied on membranes. In particular nanoparticles can be added asadditive in a polymeric membrane for example to enhance ion rejection,water reflux and compression strength of the membrane. In particularnanoparticles Polymer membranes are suitable as separation or filtrationmembrane in particular as nanofiltration membrane. In particulargraphene oxide membranes or graphene oxide coated membranes are suitableas nanofiltration membrane. Graphene oxide coated polymer membranescomprise for example polymer membranes such as polyvinylidene fluoride(PVDF), polyvinyl acetate (PVA) or polyamide (Nylon) membranes. Thenanoparticles can also be applied to ceramic and inorganic membranes,for example through solution processing or a post-production coatingprocess.

Nanoparticles according to the present invention can be used inbiomedical applications as for example as in the examples mentionedbelow:

-   -   Peptide-functionalized graphene with long time dispersion        stability in aqueous solutions as a platform to load cancer        drugs and/or RNA for therapeutic applications,    -   Graphene oxide/peptide functional complexes as a platform to        load both cancer drug and RNA,    -   Graphene oxide based scaffold to grow cells, and to study their        biocompatibility,    -   Identification of protein corona around graphene particles in        biological environment to detect cancer cells' receptors, and        use targeted delivery of antibodies.

The nanoparticles according to the present invention can furthermore beused for energy storage for example in batteries as for examplementioned in the examples below:

-   -   Graphene nanoparticles, in particular conductive graphene        nanoparticles can be used as an additive to the cathodes and        anodes of aqueous and non-aqueous lithium battery systems;    -   Graphene nanoparticles or graphene coating can be applied on the        surface of electrodes to suppress the dissolution of active        materials and/or to reduce the nucleation sites of dendrites in        the cathode and anode, respectively of the aqueous and        non-aqueous lithium battery system,    -   Graphene oxide films can be applied on the separators to        strengthen their resistance against dendrite propagation form        the anode towards the cathode,    -   Conductive current collectors comprising graphene films can be        used to reduce the size and weight of the final batteries.        Conductive current comprising graphene films can replace the        commercial current collectors comprising thick pieces of        graphite paper or comprising conductive polymer films.

In addition the nanoparticles are suitable as graphene-based dyesensitized solar cells, in energy storage devices.

Furthermore the nanoparticles are suitable for graphene-basedamphiphilic carbon nanosheets for tertiary or enhanced oil recovery.

The nanoparticles obtainable by the method according to the presentinvention are also suitable to provide quantum dots for example metallicquantum dots to improve the p-n junctions or to provide graphene quantumdots to be used in the next generation of solar cells having reducedcosts in comparison with metal based quantum dots or to providewhite-light-emitting graphene quantum dots or UV-absorbing graphenequantum dots. The dots could also be utilized in light-emitting screenssuch as televisions and computer screens.

Nanoparticles according to the present invention can be used in metaland polymer cold spraying applications for consistency in ultrasonic,supersonic, or conventional cold spraying techniques for pinhole freedeposition and consistent coating deposition on the surface.

Furthermore nanoparticles according to the present invention can be usedas adsorbents or absorbents, for example as adsorbents in miningapplications such as gold cyanidation for the leeching of gold cyanideparticles from the stripping solution or as absorbent in oil spillrecovery to absorb oil floating on water.

Nanoparticles obtainable by the mechanochemical process of the presentinvention can furthermore be suitable as transparent, conductive oranti-reflection film or coating on substrates as for example on metal ornon-metal substrates. In particular the nanoparticles are suitable astransparent, flexible conductive film for displays and electrodes.

EXAMPLES

In a first example, a sample of graphite was sonicated in a chamberusing a low-power ultrasonic probe (100 W, 30 kHz frequency) forpromoting the exfoliation of the feedstock in the presence of an ironoxide catalyst and a surfactant used for dispersion. The chamber wasenclosed with a positive pressure of CO₂.

In a second example an ultrasonic bath (40 kHz) was used to agitate thematerial of the solid feedstock (for example graphite) in the presenceof a surfactant and cobalt(II)oxide as catalyst to promote theexfoliation of the graphene sheets. The chamber was enclosed with apositive pressure of CO₂.

In a further example, a bulk mixing method was used in which the solidfeedstock (graphite) was mixed at a positive pressure of CO₂ in thepresence of a surfactant and rhenium oxide as catalyst to promote thegeneration of a bimodal distribution of the nanoparticles. One set ofnanoparticles has a particle size ranging between 50 nm and 300 nm andthe other set of nanoparticles has a particle size between 1 μm and 10μm.

In another example, a high-power sonication method was used in which thesolid feedstock (olivine) was mixed at a positive pressure of CO₂ in thepresence of a surfactant and cobalt oxide as catalyst to promote thegeneration of a bimodal distribution of the nanoparticles. One set ofnanoparticles has a particle size ranging between 50 nm and 600 nm andthe other set of nanoparticles has a particle size between 1 μm and 15μm.

In a further example, a low-power mixing method was used in which thesolid feedstock (graphite) was mixed at a positive pressure of CO₂ inthe presence of a surfactant and iron oxide as catalyst to promote thegeneration of a bimodal distribution of nanoparticles. One set ofnanoparticles has a particle size ranging between 20 nm and 250 nm andthe other set of nanoparticles has a particle size between 0.8 μm and 7μm.

Addition of Nanoparticles to Polymer Material

The effectiveness of the addition of nanoparticles in particular of GNP(graphene nanoplatelets) and GO (graphene oxide) as fillers in a polymermaterial as for example melt compounded high density polyethylene (HDPE)is illustrated below. HDPE with the addition of GNP is referred to asHDPE-GNP, HDPE with the addition of GO is referred to as HDPE-GO.

The GNP filler is obtained by the process according to the presentinvention. The GO filler was synthetized from the same GNP using amodified Hummers method. HDPE was of grade HD 6908 (ExxonMobil), ahomopolymer with a density of 0.965 g/cm³ and a melt index of 8.2 g/10min (ASTM D1238, 190° C./2.16 kg). The VTMS (vinyltrimethoxysilane)compatibilizer used for the HDPE-GNP composites was supplied by EvonikIndustries, while the maleated polyethylene (MAPE) compatibilizer usedfor the HDPE-GO composites was Epolene C-26.

The fillers were mixed with the HDPE matrix with the appropriatecompatibilizing agents and melt blended. Melt processing was performedwith a co-rotating twin screw compounder (DSM Xplore 15 mL). Matrix andfiller materials were first measured and transferred into centrifugetubes. In the case of the HDPE-GNP composites, the chosen weightfractions were 0.1, 0.5, 1.5, 5, 7, and 10 wt % GNP in HDPE. In case ofHDPE-GO composites, lower weight fractions of 0.05, 0.1, 0.25, 0.5, and1.5 wt % GO in HDPE were used. Next, an appropriate amount ofcompatibilizer was added to the mix. For the HDPE-GNP composites, 0.5 wt% VTMS (relative to HDPE) was added to the centrifuge tubes viamicropipette, and vigorously shaken manually. In the HDPE-GO blends, 25wt % MAPE (relative to GO) in pellet form was used.

These mixes were then fed into the compounder operating at 185° C. and150 rpm for melt blending. The compounder was running for 8 minutes toensure good dispersion. After melting, the blends were directlytransferred over to a 5.5 mL micro-injection mould (DSM Xplore) tofabricate samples for mechanical testing. A barrel temperature of 185°C. and mould temperature of 45° C. were used for all samples. The meltwas injected and held with a pressure of 110 bar for 1 minute. Excessplastic material from the fabricated samples was trimmed off and used assamples for differential scanning calorimetry (DSC).

The samples were characterized in terms of morphological, tensile,dynamic mechanical and thermal properties by the tests described belowto determine the effectiveness of the fillers.

Morphology

Morphological characterization of the HDPE-GNP and HDPE-GO compositeswas performed through scanning electron microscopy (SEM) with the QuantaFEG 250 ESEM (FEI). Clean cross sections for imaging were producedthrough liquid nitrogen fracturing, followed by sputter coating withplatinum to prevent charge accumulation during SEM imaging. SEMmicrographs for the HDPE-GNE and HDPE-GO composites allow to see thedegree of dispersion of the fillers in the polymer matrix and allow todetect agglomerates.

SEM micrographs for the HDPE-GNP composites show that the GNP filler waswell-dispersed in the 10 wt % composition, though many regions sawsignificant agglomeration between the platelets because of the van derWaals forces acting between them. For HDPE-GNP composites with 10 wt %GNP the multi-plate structures are clearly visible, with totalthicknesses of several hundred nm. Plate diameters vary betweenwell-dispersed regions and regions of notable agglomeration.Agglomeration was not observed in the HDPE-GNP composite with 0.1% wt %GNP, where no large plates could be found.

Compared to the HDPE-GNP composites, the HDPE-GO composites saw muchgreater agglomeration despite lower filler weight percentages beingused. As the supplied GO is expected to be under 500 nm in lateral size,agglomerates with lengths exceeding 10 μm indicates poor dispersion.This can be attributed to the fact that GO tends to be hydrophilic innature, leading to a mismatch with the highly hydrophobic polyethylenematrix. Though the agglomerates are distributed uniformly, the meltprocessing used was unable to disperse the individual GO platelets.Through visual inspection, the agglomerates present in the 0.25 wt % GOHDPE-GO composites were typically smaller than those found in the 1.5 wt% GO HDPE-GO composites.

Mechanical Properties

The mechanical properties of the composites were tested through bothuniaxial tensile testing and dynamic mechanical testing (DMA). Tensiletesting was performed with the Microtester 5848 (Instron), in accordancewith ASTM D638. The injection-moulded dogbone samples were of Type IV,as specified by the ASTM standard. Samples were then loaded in uniaxialtension at a rate of 50 mm/min until either failure, or the machinereached its physical limit. Seven samples were tested for eachcomposition, from which their Young's modulus, peak stress, modulus ofresilience, and elongation strains at break were calculated andanalyzed. DMA testing was conducted with the DMA Q800 (TA Instruments),in which the thin rectangular beam samples were loaded into adual-cantilever fixture and subjected to a cyclic three-point-bendingtest. Samples were tested with a linear frequency sweep from 0 to 60 Hzat three different oscillation amplitudes (30, 60 and 120 μm) at 35° C.From this testing, their storage moduli, loss moduli, and loss tangentscould be obtained.

The tensile properties of the HDPE-GNP and HDPE-GO composites are shownin FIG. 2 : A and B show the Young's modulus of respectively HDPE-GNPand HDPE-GO composites with different concentration of GNP and GO, C andD show the peak stress of respectively HDPE-GNP and HDPE-GO compositeswith different concentration of GNP and GO, E and F show the resilienceof respectively HDPE-GNP and HDPE-GO composites with differentconcentration of GNP and GO, and G and H show the maximum elongation ofrespectively HDPE-GNP and HDPE-GO composites with differentconcentration of GNP and GO.

Tensile testing showed notable improvements in Young's modulus for boththe HDPE-GNP and HDPE-GO composites, despite significant agglomerationin the higher loadings as evidenced by the SEM imaging. The maximummeasured Young's modulus occurred in the 10 wt % GNP HDPE-GNP composite,in which the increase in Young's modulus over neat HDPE was over 55%. Asmaller increase of 22% in the peak stress was also seen.

In the case of HDPE-GNP composites, a general upward trend in elasticmodulus and peak stress is observed with increasing filler loading.However, 0.1 wt % GNP loading marks a peak for Young's modulus andmaximum stress, which is followed by local minima for modulus andstrength at 0.5 wt % and 1.5 wt %, respectively. This suggests that thetotal degree of reinforcement is dependent on multiple factors. Morespecifically, at very low GNP loadings, dispersion is excellent andagglomeration is not an issue, resulting in the good reinforcement seenat 0.1 wt % loading. With increasing GNP loading, the amount ofagglomeration will increase, leading to less effective reinforcementdespite the greater abundance of reinforcing fillers. This was reflectedin the drop in Young's modulus from 0.1 wt % to 0.5 wt %, and a similardecrease in peak stress from 0.1% to 1.5%. Increasing filler loadingfurther, the agglomeration sizes will reach a limit due to the shearpresent in the compounder, and thus the filler quantity becomes thedominant variable, which would resume the positive trend in both modulusand tensile strength. In terms of strain to break, the HDPE-GNPcomposites saw a marked decrease with greater filler loading. It shouldbe noted that the neat HDPE specimens were extended until around 200%elongation, reaching the mechanical limit of the machine beforebreaking. The modulus of resilience was calculated by integrating thestress-strain curves from zero strain until peak stress, ignoring thelatter plateau in the curves. For the HDPE-GNP composites, the lowerloadings exhibited resilience comparable to that of neat HDPE, while thehigher loadings demonstrated progressively lower resilience, owing tothe reduction in material compliance with increasing filler loading.

The HDPE-GO composites exhibited less prominent trends, due to thenarrower range of loadings tested. Nonetheless, an increase in bothmodulus and peak stress over all the loadings as compared with neat HDPEwas observed. The most noticeable increase in the modulus and peak wasfrom neat HDPE to 0.05 wt %, showing 31.6% and 10% increasesrespectively. Between 0.05 wt % GO and the subsequent three loadings,modulus and peak stress values were all measured to be within onestandard deviation of each other, indicating an asymptote origin ataround 0.05 wt % respectively. As seen from the SEM images, the GOparticles within the HDPE matrix demonstrated significant agglomerationat all weight contents. It is believed that the addition of 0.05 wt %increased the properties by promoting the required shear transfer to thefillers. However, increasing the filler content results also in increasein the agglomerate sizes resulting in a competition between reinforcingrole of the fillers and the loss in load bearing due the increase in theagglomerate sizes. The strain at break shows a decreasing trend withincreasing filler content.

The dynamic mechanical characteristics of the HDPE-GNP and HDPE-GOcomposites are presented in FIG. 3 : A and B show the storage modulus ofrespectively HDPE-GNP and HDPE-GO composites with differentconcentrations of GNP and GO, C and D show the loss modulus ofrespectively HDPE-GNP and HDPE-GO composites with differentconcentrations of GNP and GO, E and F show the loss tangent ofrespectively HDPE-GNP and HDPE-GO composites with differentconcentrations of GNP and GO.

For both HDPE-GNP and HDPE-GO composites, an increase in storage modulusis generally observed with increasing filler loading. These measurementsmatch closely with the elastic modulus results obtained from tensiletesting. Examining loss modulus, values increased with greater fillerloading as the resulting increase in stiffness inherently leads to ahigher effective viscosity in the material, according to theKelvin-Voigt model

Examining loss tangent, the lowest amount of viscoelastic losses wasperceived in neat HDPE and generally increased with higher fillerloadings. This may be attributed to the relatively weak interfacialbonding between filler and matrix, which will act as frictionaldissipation mechanisms through stick-slip motion between filler andmatrix. Frictional losses would also be more prevalent in compositeswith higher filler loadings where agglomeration is more widespread, inwhich the weak filler-filler interactions will introduce additionaldegrees of freedom for sliding and rotation in the composite in additionto those present in the filler-matrix interfaces. These dissipativeforces would in turn be measured as further viscous losses. In allcases, the loss tangent was extremely low, indicating that the HDPEmatrix is not well-suited for damping applications.

Thermal Properties

The crystallization and melting characteristics of the two compositeswere determined through differential scanning calorimetry (DSC).Specifically, the DSC Q2000 (TA Instruments) was utilized, runningcomposite samples through a heat-cool-heat cycle from ambient roomtemperature (22° C.) to 180° C., down to −40° C., and back to 180° C.,all at a rate of 10° C./min. Samples consisted of thin slices ofcomposite ranging from 10-12 mg in mass.

The melt and crystallization behaviour of the HDPE-GNP and HDPE-GOdetermined by DSC are displayed in FIG. 4 and Table 1.

TABLE 1 Melt and crystallization points, and percent crystallinity forthe HDPE-GNP and HDPE-GO composites Crystallization Melting TemperatureTemperature Crystallinity Composition (° C.) (° C.) (%) Neat HDPE 135.95± 0.33 119.34 ± 0.32 65.12 ± 0.82 0.1% GNP 133.69 ± 0.04 120.64 ± 0.1756.10 ± 0.39 0.5% GNP 134.31 ± 0.26 121.31 ± 0.17 53.85 ± 0.10 1.5% GNP134.29 ± 0.12 120.88 ± 0.28 58.35 ± 0.73 5% GNP 133.86 ± 0.35 121.11 ±0.02 60.00 ± 0.32 7% GNP 133.69 ± 0.19 122.20 ± 0.01 59.97 ± 0.44 0.05%GO 134.10 ± 0.04 119.00 ± 0.50 52.52 ± 1.25 0.1% GO 134.61 ± 0.28 119.52± 0.26 55.69 ± 0.97 0.25% GO 133.91 ± 0.01 118.58 ± 0.29 56.46 ± 0.700.5% GO 133.82 ± 0.06 118.92 ± 0.53 57.76 ± 0.86 1.5% GO 133.57 ± 0.12119.81 ± 0.07 58.14 ± 1.73

DSC analysis showed a decrease in melting temperature for both theHDPE-GNP composites and the HDPE-GO composites relative to neat HDPE.This phenomenon may be attributed to the disruptive effect that GNP andGO fillers have on the HDPE polymer chains. By restricting polymer chainmovement and promoting the formation of smaller crystallites, theaddition of fillers slightly reduces the melting temperature of thepolymer composite. On the cooling curves, an increase in crystallizationtemperature was observed in the HDPE-GNP composites, but generally notso for the HDPE-GO composites. The increase can be explained by thetendency of fillers to promote heterogeneous crystal nucleation inpolymers, as long as the filler is small enough in size andwell-dispersed. In the case of the HDPE-GO composites, agglomeration waspresent to a greater extent compared to the HDPE-GNP composites anddispersion was inconsistent, in turn leading to fewer nucleation sites.As such, there were no clear trends in crystallization temperature forthe HDPE-GO composites tested.

The crystallinity of the composites was computed based on the enthalpyof fusion exhibited during melting in the initial heating curve of theDSC test. The formula used was the following:

$\begin{matrix}{X_{c} = {\frac{\Delta H_{f}}{\Delta{H_{f}^{o}\left( {1 - W_{filler}} \right)}} \times 100\%}} & (1)\end{matrix}$

where X_(c) is the degree of crystallinity of the material, ΔH_(f) ^(o)is the enthalpy of fusion in melting for a theoretical 100% crystallineHDPE matrix, ΔH_(f) is the measured melting enthalpy of the testmaterial obtained by the linear peak integration method in the DSCsoftware, and W_(filler) is the weight fraction of GNP or GO used in thetest. From literature, ΔH_(f) ^(o) for HDPE was found to be 293 J/g. Thecrystallinity calculations for the set of composites showed that neatHDPE had the highest crystallinity at 65%, while the composite samplesfor both the HDPE-GNP and HDPE-GO composites measured between 52% and60% crystallinity. This phenomenon can again be explained by thetendency of fillers to suppress crystal growth in certain polymermatrices. As neat HDPE already exhibits relatively high crystallinity,the inclusion of GNP or GO serves more to reduce free volume and hinderpolymer chain mobility, thus discouraging the formation of larger, moreordered crystals. Nonetheless, the higher filler loading compositesgenerally showed higher crystallinity relative to the lower loadings,due to the difference in nucleation potential arising from an increasein the sheer number of nucleation sites in the composite.

Toxicity of Graphene Oxide Nanoparticles

Different graphene derivatives were compared in a toxicity study.HCT116, a colon cancer cell-line, was selected as the target cells.These cells were incubated with various concentrations of a number ofgrapehene derivatives, notably graphene oxide nanoribbon (GONR), reducedgraphene oxide (RGO), a graphene oxide produced (using an acid treatmentprocess) from nanoparticles of the present invention (GO), milledgraphite with 10% CO₂ uptake and graphene oxide nanocaps (GONCs). Thesederivatives are different in size, structure and surfacefunctionalities. The results show that all graphene samples become moretoxic (less viability %) to the cells by increasing their concentrationin the cell medium (μg/mL). However, the GO sample showed the minimumtoxicity impact on the cells even at the very high concentrations(400-500 μg/mL). While the typical graphene oxide species, reported inthe literature, have been identified to be toxic even at lowconcentrations of 200 μg/mL, this unique GO has shown the lowesttoxicity, which will provide many potentials for biomedicalapplications. This unique GO was obtained through chemical treatment ofa milled graphite (10%).

Zeta of Graphene Oxide Nanoparticles

Zeta potential is a measure of the magnitude of the electrostatic orcharge repulsion/attraction between particles, and is one of thefundamental parameters known to affect stability. In fact, it is theelectric potential in the interfacial double layer (DL) at the locationof the slipping plane relative to a point in the bulk fluid away fromthe interface. In other words, zeta potential is the potentialdifference between the dispersion medium and the stationary layer offluid attached to the dispersed particle. Its measurement bringsdetailed insight into the causes of dispersion, aggregation orflocculation, and can be applied to improve the formulation ofdispersions, emulsions and suspensions. Table 2 below shows the zetapotential values for a wide range of graphene derivatives with differentsize, structure and functionalities. GO, the graphene oxide produced(using an acid treatment process) from nanoparticles of the presentinvention, has a Zeta potential of −52 mV and has the highest dispersionstability, while RGO has the lowest stability with a Zeta potential of−22 mV.

TABLE 2 Zeta potential measurements of different graphene derivatives.Zeta Graphene potential Name Particle size (mV) Description GONR 200 nmwidth, −50 Graphene oxide 500 nm length nanoribbon RGO 200 nm lateralsize, −22 Reduced graphene 5 nm thickness oxide by hydrazine from CUT-GOGO 200 nm lateral size, −52 Graphene oxide from 1 nm thickness GNPMilled 1 um lateral size, −41 Milled graphite Graphite 10% 50 nmthickness nanoplatelet GONC 30 nm lateral size, −44 Graphene oxide 1 nmthickness nanocaps

1. A mechanochemical process to produce exfoliated nanoparticles, saidmethod comprising the step of: subjecting the material of a solidfeedstock in the presence of a gas comprising carbon dioxide to amechanical agitation operation in a mechanical agitation unit at apressure of at least 1 atm (15 psi), wherein the solid feedstockcomprises an inorganic amorphous powder.
 2. A mechanochemical processaccording to claim 1, wherein said oxidizing gas comprises a gasselected from the group consisting of oxygen, sulfur dioxide, nitrogendioxide, and combinations thereof.
 3. A mechanochemical processaccording to claim 1, wherein said mechanochemical agitation operationcomprises: mixing, stirring, shearing, shaking, blending orultrasonication.
 4. A mechanochemical process according to claim 1,wherein said process further comprises the addition of a catalyst tosaid solid feedstock.
 5. A mechanochemical process according to claim 4,wherein said catalyst comprises a metal oxide selected from the groupconsisting of iron oxides, cobalt oxides, rhenium oxides, titaniumoxides and combinations thereof.
 6. A mechanochemical process accordingto claim 1, wherein said process further comprises the step ofintroducing at least one intercalant agent.
 7. A mechanochemical processaccording to claim 6, wherein said intercalant agent comprises an acidselected from the group consisting of hydrochloric acid, sulfuric acid,nitric acid and combinations thereof.
 8. A mechanochemical processaccording to claim 5, wherein the catalyst is added through a lining onthe inside wall of the mechanical agitation unit.
 9. A mechanochemicalprocess according to claim 1, wherein said oxidizing gas comprisescarbon dioxide gas emissions derived from industrial processes.
 10. Amechanochemical process according to claim 1, wherein said oxidizing gascomprises carbon dioxide at a level within the range of 70-95%.